Frequency-phase converter with inductive current control



July 13, 1954 J. R. PARSONS FREQUENCY-PHASE CONVERTER WITH.;

INDUCTIVE CURRENT CONTROL Filed April l5, 1949 l0 Sheets-Sheet l Fig. l. i STU Conductive Thyratron or lgnitrons Anode .g

Potential E 2 Firing Curve o Bias Thru i v f, 2 359 Time. I i 455 *\Bios8Thru Blas 539 Tnyrglrgg 3 9 Fig.6.

(PotentiaI Thru INVENTOR Potential Thru 23 26 2|,22,24,25 John R. Parsons.

ATTO RN EY July 13, 1954 J. R. PARSONS FREQUENCY-PHASE CONVERTER WITH INDUCTIVE CURRENT CONTROL Filed April l5, 1949 lO Sheets-Sheet 2 I q I E @f l -N I0 I l ...l z

'' INVENTOR John R.Parsons. BY 979mm www( ATTORNEY July 13, 1954 J. R. PARSONS 2,683,851

FREQUENCY-PHASE CONVERTER WITH INDUCTIVE CURRENT CONTROL Filed April l5, 1949 10 Sheets-Sheecl 3 INVENTOR John RPursons.

w BY

ATTORNEY July 13y 1954 J. R. PARSONS FREQUENCY-PHASE CONVERTER WITH INDUCTIVE CURRENT CONTROL l0 Sheets-Sheet 4 Filed April l5, 1949 INVENTOR John R.Porsons.

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ATTORNEY July 13, 1954 .1. R. PARsoNs 2,683,851

FREQUENCY-PHASE CONVERTER WITH INDUCTIVE CURRENT CONTROL Filed April l5, 1949 l0 Sheets-Sheet 5 Hmm() uguguuawg ouanbaxj lg Jauxgl aouanbas www louuoo INVENTOR John R. Parsons.

ATTORNEY July 13, 1954 J. R. PARSONS 2,683,851

FREQUENCY-PHASE CONVERTER WITH INDUCTIVE CURRENT CONTROL lO Sheets-Sheet 6 Filed April l5, 1949 l .mai

om l am Bm mom mom INVENTOR John R. Parsons.

ATTORNEY July 13, 1954 J. R. PARSONS FREQUENCY- PHASE CONVERTER WITH INDUCTIVE CURRENT CONTROLJ 10 Sheets-Sheet 7 Filed April l5, 1949 w m mda July 13, 1954 J. R. PARSONS 2,683,851 FREQUENCY-PHASE: CONVERTER WITH INDUCTIVE CURRENT CONTROL Filed April l5, 1949 10 Sheets-Sheet 8 Fig.8.

Control Circuit Sequence Timer 8| Frequency Determining Circuit Control Circuit Sequence Timer Si Frequency Determining Circuit witnesses: I INVENTOR Flg-9- John R. Parsons.

ATTORNEY July 13, 1954 Filed April 15, 1949 INDUCTIVE CURRENT CONTROL l0 Sheets-Sheer?I 9 n@ T @a f tgl w Q 5% i .N LL

` WITNESSES: INVENTOR @a @WM John R. Parsons.

ATTORN EY lO Sheets-Sheet l0 J. R. PARSONS FREQUENCY-PHASE CONVERTER WITH INDUCTIVE CURRENT CONTROL 1949 July 13, 1954 Filed April 15 Patented July 13, 1954 FREQUENCY-PHASE CONVERTER WITH INDUCTIVE CURRENT CONTROL John E. Parsons, Kenmore, N. Y., assignor to Westinghouse burgh, Pa.,

Electric Corporation, East Pittsa corporation of Pennsylvania Application April 15, 1949, Serial No. 87,714

(Cl. S21-Jl) Claims. 1

My invention relates to electric discharge apparatus and it has particular relation to resistance welding apparatus. Application Serial No. 384,703 is a division of the present application. The invention is an outgrowth of my experience with low frequency welding apparatus of the type disclosed in the following patents, all of which are assigned to Westinghouse Electric Corporation:

I. R. Parsons.

The above-listed applications relate to welding systems in which power is derived from an alternating current commercial supply and supplied to a load such as a welding transformer at a substantially lower frequency than that of the supply. The supply may be of the singlephase or polyphase type. The conversion from the commercial frequency to the lower frequency is eected by operation of electric discharge valves, usually ignitrons, interposed between the supply buses and the primary of the welding transformer. Preferably, the ignitrons are subdivided into pairs, each pair being connected in anti-parallel to a section of the primary of the Welding transformer and each section and its associated pairs being connected across a pair of buses of the supply. The individual ignitrons of the pairs are so controlled that first current of one polarity is built up in the Welding transformer and permitted to decay, and immediately thereafter current of the opposite polarity is built up and permitted to decay. The ignitrons of each pair which conduct the current of each polarity I will designate a group. In the systems shown in Patents 2,640,180, 2,619,591 and 2,508,467 there are three ignitrons in each group. In the system shown in Patent 2,510,652 there are two ig'nitrons in each group.

In operating apparatus of this type, I have found that it is on the whole satisfactory. I have, however, encountered situations in which certain of the ignitrons have a relatively short life in operation. In such situations the ignitrons are severely damaged after the apparatus is operated for only a short time; sometimes the damage is so severe that the ignitrons must be replaced. In addition the load may also be damaged. In particular, I have found that such difficulties arise when the apparatus is operated with a load having a low power factor such as arises when the welding electrodes are shortcircuited for test purposes.

It is, accordingly, an object of my invention to provide a low frequency welding system of the electronic type in which the electric discharge valves conducting the load current shall operate without becoming seriously impaired in a short time.

Another object of my invention is to provide welding apparatus including electric discharge valves through which the load current is supplied which shall operate at a low power factor, such as is encountered when the welding electrodes are short-circuited, without serious impairment to the discharge valves.

stru another object of my invention is to pro- Vide an electronic control system for converting power derived from a commercial supply to a substantially lower frequency to be supplied to a load such as o. welding transformer which shall operate over long periods of time without serious impairment to the electric discharge valves which conduct the load current.

A further object of my invention is to provide an electronic converter for deriving power from an alternating supply at a substantially lower frequency than that of the supply which shall operate to supply loads having power factors of the order of 20% without serious damage to the electric discharge valve which carry the load current.

A still further object of my invention is to provide an electronic converter for deriving power from an alternating supply at a substantially lower frequency than that of the supply which shall operate without damage to the load.

An ancillary object of my invention is to provide a novel control circuit for controlling the iii-ing of the main discharge valves in systems such as are shown in the above-listed applications.

Still another ancillary object of my invention is to provide a converter 0f the type disclosed in the above-entitled Patents 2,640,180, 2,519,591 and 2,508,467 which shall include a substantially smaller number of main valves and control circuits for main valves than do the corresponding systems disclosed in the above-mentioned three applications and shall, for many purposes, afford as satisfactory service as the apparatus disclosed in said three applications.

My invention arises from the realization that one of the ignitrons of each of the groups of ignitrons included in the systems disclosed in the above-listed applicationsfails to become extinguished at the end of its conductive period and continues to conduct for time intervals of suhstantial duration. It is these ignitrons which are damaged.

In operation of the systems disclosed in the above-listed applications, one ignitron of each pair of anti-parallel connected ignitrons, that is of each group, is rendered conductive when the bus to which its anode is connected becomes positive relative to the other buses, and conducts in its turn during a low frequency half period until the current built up in the load has the desired low frequency amplitude. The last ignitron of the group to conduct carries the masimurn current. When this current is reached, the first ignitron of the conducting group is not again fired. As the current decays in the load, the'last ignitron of the group continues to conduct. The current during the low frequency half period of the opposite polarity is now to be conducted by the other ignitrons of each pair. Each of the latter is rendered conductive in its as the bus to which its anode is connected becomes more positive than the other 'cuses of the supply. I have found that the damage to the ignitrons arises from the fact that when the first of the ignitrons which conducts the current of the reverse polarity is rendered conductive, the c last of the ignitronsJ which conducts the current of the initial polarity is still conductive. The current conducted by the nrst ignitron to conduct the reverse current induces potential in the winding supplied from the last ignitrcn to conduct the current of the initial polarity in such a sense as to build up the decaying conductivity of this last ignitron. 'Ihe latter, therefore, conducts for an excessively long interval. In addition the current supplied to its corresponding winding through the first ignitron of the new group is returned to the supply through the last ignitron of the old group. A short circuit is thus in effect produced. Because the last ign' ron of the old group conducts for an excessively long interval under short circuit conditions it soon becomes hot, fails to deionize and carries current of both polarities continuously. The same condition arises as the last ignitron to conduct the reverse current continues conductive after the first ignitron to conduct the current of the initial polarity is rendered conductive during the succeeding low frequency half period. This phenomenon is more common at low power factors than at high power factors, and is particularly marked when the apparatus is operated with the welding electrodes short-circuited for test purposes.

In accordance with my invention, I provide apparatus in which the firing of the last of the valves of each group to conduct is initiated later' in the periods of the supply than is the firing of the other valves. "fhis delay may, in accordance with my invention, be introduced during each of the successive intervals during which this valve conducts during each low frequency half period, or it may be introduced only during one, preferably the last, of the intervals. In accordance with the broader aspects of my invention, this delay may also ce introduced into one of the other valves than the last, or into several of the valves. By reason of the delay in the firing of the last valves to conduct, the build-up of the current in the load is somewhat reduced and the current through the last valve to conduct is ii-- terrupted before the first valve to conduct the el current of the reverse polarity becomes condrctive.

In accordance with another aspect of my invention, I provide a system in which the load is supplied from less than all of the phases of a polyphase supply. For example, the load may be supplied from only two phases of a three-phase supply. Under such conditions, the current in the load is built up in such manner that the excessive conductivity by certain valves described above is also avoided.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will be understood from the following description of a specific embodiment when read in connection with the accompanying drawings, in which:

Figure 1 is a graph illustrating the operation of prior art apparatus shown in Figs. 1A, IB and 1C;

Figs. 1A, 1B, and 1C are Figs. 1, 2 and 3 of Patent 2,619,591 and are incluf1 to facilitate understanding of the invention hereof;

Fig. 2 is a circuit diagram o' a preferred embodiment of my invention;

Fig. 3 is a graph illustrating the apparatus shown in Fig. 2;

Fig. 4 is a graph analogous to the graph shown in Fig. 1, but corresponding to the shown in Fig. 2;

Figs. 5 and 5A together constitute a circuit diagram of a modication of my invention;

Fig'. 6 is a graph illustrating the operation of the apparatus shown in Figs. 5 and 5.a;

Fig. 7 is a graph similar to the graphs shown in Figs. 1 and 4, but corresponding to the apparatus shown in Figs. 5 and 5A;

Fig. 8 is a circuit diagram of a further embodiment of my invention;

Fig. 9 is a circuit diagram of still modification of my invention;

Figs. l0 and 19A together constitute a circuit diagram of a still further modification; and

Fig. 11 is a graph similar to the graph shown in Figs. 1, 4 and 7, but corresponding to the modiiications shown in Figs. 1c and 10A.

For a proper understanding of my invention, it is desirable that the operation of the apparatus shown in any of the above-listed applications be understood. To facilitate the explanation of my invention, I shall, accordingly, brieiiy discuss the operation of the apparatus shown in Figs. l, 2 and 3 of Patent 2,619,591.

The apparatus shown in Patent 2,619,591, includes a welding transformer having three primary windings li, i2 and i3. The primaries are supplied from the buses Ll, L2, L3 of a threephase source through three pairs of ignitrons, iTU, ZTU; ETU, flTU; and 'U, GTU cach connected in anti-parallel. rEhe buses Li, L2, LS are customarily t -e buses of a commercial alternating current supply which in the United States has a frequency of 60 cycles and in other countries may have a different frequency, such as 5c cycles, for example. Power is supplied to the operation of the primaries Il, i2 and i3 in sequence, first in one direction during one low frequency half period through ignitrons iTU, STU and ii-TU constituting one group, and then during the succeeding low frequency half period in the opposite direction through ignitrons 2'IU, LTU and STU constituting another group.

When ignitrons ITU, STU and STU are conductive, current ows from right to left through each ofl the primaries II, I2 and I3, and when ignitrons ZTU, 4TU and STU are conductive, current flows from left to right. Each of the ignitrons becomes conductive when the bus to which its anode is connected is positive relative to the other buses and the proper control potential to fire its firing thyratron is impressed. The instant when an ignitron becomes conductive is determined by the setting cf the taps 4s on the rheostat 3S. The ignitrons ITU, 3TU and STU, or 2TU, 4TU and BTU, respectively, conduct in their turn during a predetermined number of successive periods of the supply which is determined by the low frequency which is desired. For example, we may assume that each of the ignitrons conducts twice during each low frequency half period of the supply. Under such circumstances, the low frequency is of the order of 12 cycles per second.

The apparatus described, on the whole, operates satisfactorily; but at low power factors the last of each of the ignitrons to conduct, that is, the ignitrons STU and iTU may continue to conduct indefinitely. This operation is illustrated in Fig. 1 of the present application. In this view, the current built up in the welding transformer T is plotted as a function of time. Current is plotted vertically .and time horizontally. To facilitate understanding equally spaced abscissae, corresponding to instants when successive ignitrons `become conductive, are labeled in terms of twelfths of periods of the supply. Each of the loops represents an interval during which one of the ignitrons of a group is conductive and is 5 labeled accordingly.

During the first low frequency first half period, each of the ignitrons ITU, STU and STU is conductive twice. During the first #E of a supply period, ignitron ITU conducts current from right to left through winding I I. During this interval bus LI is positive relative to the other buses. At the end of the interval bus L2 becomes positive with respect to LI, but ignition ITU continues to conduct because of the back potential provided by the winding II. Now while ignitron ITU is still conductive, ignitron STU is rendered conductive and current flows from right to left through winding I2. This current flow induces a. potential across winding I I which is so poled as to cause its right-hand terminal to become substantially positive with respect to its left-hand terminal. Ignitron ITU is therefore extinguished. In the same manner as represented by their corresponding loops, ignitrons STU, ITU, STU and STU are each in its term rendered conductive extinguishing the previously conductive ignitron. When ignitron STU is conductive for the second time, the ignitron ITU is not again rendered conductive. The current flow through ignitron STU and its associated winding I3 then decays as shown by the sixth loop STU of the left-hand half wave of the curve. At this time the potential of the anode of ignitron STU is equal to the potential of bus L3, and the potential of its cathode is equal to the potential of bus LI less the counteracting potential produced by winding I3. As the current flow through the ignitron STU is decaying, the potential of bus LI is becoming more positive than the potential of bus L3 and this inverse difference of potential tends to extinguish ignitron STU. The ignitron STU is thus maintained conductive by the back electromotive force produced by the decay of flux in the transformer T, which is CFI manifested as a potential across primary I3 in' such a sense that the left-hand terminal of this primary is positive and the rig t-hand terminal is negative. If the current flow through ignitron STU would be permitted to decay uninterruptedly, this potential across primary I3 eventually would become smaller than the negative potential between buses LI and L3, and the ignitron STU would become non-conductive.

However, -lf of a period after ignitron STU was rendered conductive, ignitron 2TU is rendered conductive as represented by the first loop 2TU of the right half wave. As represented by the overlapping portion of the curve, the ignitron 2TU sometimes becomes conductive before the conductivity of ignitron STU is interrupted. This condition occurs particularly when the power factor of the load is low-of the order of 20% when calculated on the basis of GIB-cycle supply. Ignitron 2TU conducts current from left to right through the primary I I and induces a potential in primary I3 which increases the positive potential of its left terminal relative to its right terminal. This potential adds to the potential arising from the decaying flux and tends to maintain ignitron STU conductive as represented by the rising tail R. on the positive curve. Power derived from the supply through ignitron ZTU between positive bus L2 and negative bus L3 is returned to the supply through ignitron STU between negative bus L3 and positive bus LI. In effect the supply is thus short circuited. The potential induced from primary I I increases as represented by the first negative loop until ignitron ITU becomes conductive when current flows from left to right through primary I2 as represented by the second negative loop. Again, a potential tending to maintain ignitron STU conductive, is induced and the latter, if it is still conductive, continues to conduct. Eventually, ignitron STU is rendered conductive as represented by the third negative loop and ignitron STU, if it is still conductive, is extinguished. However, ignitron STU has conducted for an efrcessively long interval under short-circuit conditions and eventually becomes so hot as to conduct continuously as the above-described process is repeated during successive welds. The shortcircuit operation also materially deteriorates the transformer T. The above-described operation also occurs when ignitron STU fails to become non-conductive before ignitron ITU becomes conductive.

The apparatus shown in Fig. 2 includes a welding transformer ll of the type disclosed in the Patent 2,619,591. This transformer has three primaries 33, 36S and 331. Each of the primaries is connected between a pair of buses M9, 3II; 3II, 3I3; and 3I3, 389, respectively, of a three-phase power supply' through a pair of antiparallel ignitrons SIS, 3I1; 3I5, 32|; and 323, 32S, respectively. Thyratrons 321, 329, 33I, 333, 335 and 331, respectively, are provided for firing each of the ignitrons. In the control circuit of each of the thyratrons, a bias is impressed from transformers 339, 34H, 343, 3dS, 36'! and 343, respectively. The primaries 35|, 353, 35S and 3S? of the transformers 339 to 3&5, respectively, associated with these four firing thyratrons 321 to 333, respectively, are connected across tivo of the buses 369 and 353 through a resistor 35. The secondaries 3M, SSS, 38S and 331 of each of the transformers 339 to 35.5, respectively, are each connected across capacitors 369, 31I, 313 and 315, respectively, through rectiers 311, 319, BBI

:accessi :and 333, .1-espectively, preierably @of the f dry type.

Capacitors 353 to 315, respectively, are connected in the control circuit of its thyratrons 321 to 333, respectively, in such-.a sense that the charge which is impressed on it from its transiormers 333 to 335 tends to .maintain the thyratron non-conductive. Other biasing potential from the primaries 385 and 381 of the other transformers 331 and 335 are connected across the buses .353 and 3|3 through a second resistor 339 which is of substantially smaller magnitude than the first-named resistor. The secondaries 33| and 333 or" these latter transformers are connected to charge capacitors 335 and 391 in the same manner as the secondaries of the other transformers. The biasing transformers 339 to 345, the capacitors 333 to 313, the transformers .331 and 333, and the rectiners 311, 313, 33|, 333,

393 and 43| are for all thyratrons, substantially identical. Accordingly, the biasing potential impressed in thyratrons 335 and 331 is substantially greater than the corresponding bias for the other thyratrons 321 to 333.

The firing thyratrons are controlled from a Control Circuit 332 identical to 'that shown in Fig. 2 of application Serial No. 52,103. The conductor pairs labeled 2|, 22, 23, 213, 25 and 25 each connected, respectively, to the control electrode 333, 335, 431, 533, 3H, M3, respectively and to the biasing capacitor 369, 313, 395, 3"l|, 315, and 331, respectively, of each of the thyratrons 321, 33|, 335, 323, 333 and 331, respectively, are identical to the conductor pairs 2|, 22, 23, 23, 25 and 25 shown in Fig. 2 of application Serial No. 52,103. The Control Circuit is controlled from a Sequence Timer and Frequency Determining Circuit fil-5, such as is shown in, Fig. 3 of the application Serial No. 52,103. The conductor pairs labeled 35 and 33 in the Fig. 2 of the present application, are identical to the conductors labeled 56 and 33 in Fig. 3 of the above-mentioned application.

In the quiescent state of the apparatus, the firing thyratrons 321 tc 331' are maintained nonconductive by the biases derived from the capacitors 333 to 315 and 395 and 391, and the ignitrons 3 l5 to 325 are also non-conductive. In operation, pulses are impressed at the beginning of a number of successive periods of the supply rst through conductors 36, and then, preferably during an equal nun ber of periods, through conductors 3G. The pulses impressed Athrough conductors 3 causes pulses to be impressed in succession through conductors 2|, 22 and 23. Each pulse from conductors 35 produces three pulses in succession, the first through conductors 2|, the second through conductors 22 and the third through conductors 23. The latter pulses are impressed during the time when the bus '333, 3| I or 3 i3 connected to the anode of the ignitron 3|5, 3i'3 or 323, respectively, associated with the conductors 2|, 22, and 23, respectively, is positive relative to the other buses and preferably just as the respective buses become positive. These pulses have the wave form of half waves of the power supply and amplitudes sunicient to counteract the biases derived from the capacitors 335, 313 or 335, respectively. The higher bias impressed on the thyratron 335 is counter-acted later in the corresponding half periods by the pulses derived from the conductors 23 than are the biases impressed on the other thyratrons 321 and 32| by the pulses impressed through the conductors 2| and 22, respectively. The pulses impressed through conductors 83 produce pulses yin `succession through conductors 24, 25 and 25; each pulsethrough conductor 33 producing three pulses, theiirst through conductor 23, the second through 25 and the third through 26. The latter pulses are preferably impressed just as the buses 3| 313 .or` 335 connected to the anodes o1" the associated ignitrons `3l-1, 32| or V325 become positive.

This operation is illustrated in Fig. 3 in which potential is plotted vertically and time horizontally. The upper half sine wave represents a potential such as one derived from two or" the buses 335, 3H, 3|3 of the power supply and impressed as vanode potential across one o the firing thyratrons. The shallow curve represents the critical firing curve corresponding to the anode potential represented by the upper curve. The intermediate line below the time axis represents the bias of lower absolute magnitude impressed through the resistor 353 in the control circuits of thyratrons 321 to 333 and the lowest line represents the bias of the higher magnitude impressed through the resistor 333 in the control circuit of the other thyratrons 335, 321. The counteracting potential derived from the conductors 2| and 22, and 2li and 25, is represented by the half wave on the intermediate line and the counteracting potential derived from the conductors 23 and ls represented by the half wave on the lowest line. The latter half wave intersects the critical curve substantially later in the anode half period than the former wave. Therefore, the iiring thyratrons 321 to 333 which have the lower bias impressed in their control circuits and their associated ignitrons 315 to 32| are red earlier than the other thyratrons 335 and 331 and their associated ignitrons 323 and 325.

As the pulses are impressed in the control circuits of the thyratrons 321, 33| and 335, rst from conductors 2|, then from conductors 22, then from conductors 23, each or the ignitrons -3|5, 3|9 and 323 is fired in its turn. First current is conducted through the iirst ignitron 3|5 land through its associated primary 333, next current is conducted through the second ignitron 3|3 and its associated primary 335, and finally, at a longer interval than the interval between the rst two ccnducting operations, current is conducted through the third ignitron 323 and its associated primary 331. This process is repeated so long as pulses are supplied through conductors E6, each of the ignitrons 3|5, 3H) and 323 conducting in its turn a number of times determined by the frequency of operation desired. Thereafter, pulses are impressed through conductors 33, and the other ignitrons 3H', B2i and 325 conduct each in its turn. Current oi a polarity opposite to the 'first polarity now -iicws through the primaries 353, 335 and 33?. In this case also the delay between 'the conduction ci the second and third ignitrons 32| and 325, respectively, exceeds the delay between the conduction of the first and second ignitrons 3|? and 32|, respectively.

Because of the delay between the conduction of the second and third ignitrons 3|13 or 32| and 323 or 325 in each case, the current flow through the load 33| which is conducted by 'the second ignitron `3|3 or 32| decays somewhat before the third ignitron 323 or 325 becomes conductive.

'The current through load 35|, therefore, builds one polarity is therefore interrupted before the rst ignitron 3l7 or 3|5, respectively, to conduct current if the opposite polarity is rendered conducitve.

This operation is illustrated in Fig. 4 in which the current flow through the transformer 39% is plotted vertically and timed horizontally. For the purposes of plotting the graphs, have as sumed that during each low frequency period, each of the ignitrons 3i5 to 325 conducts twice. As indicated, each of the loops in the left-hand portion of this graph represents the current conducted by one of the ignitrons 3 l5, 3 l@ or 323 which conducts current of one polarity the loops in the right portion represent the current conducted by the other ignitrons 3|7, 324 and 325. The second and the fifth loops SlS 32| of each half period are longer than the others and drop off slightly because of the delay in eac case in the firing of the ignitrons 323 and 325. The tail on the sixth loop 323 of the left portion reaches the time axis to the left of the beginning of the first loop 3|7 of the second portion; this representation corresponds to the termination in the conductivity of ignitron 323 before ignitron 3 i7 is fired. The overlap of Fig. l does not occur.

The apparatus shown in Figs. and 5A includes a transformer 33|, ignitrons 3| 5 to 325 and firing thyratrons 327 to 337 which are similar to those of the system shown in Fig. 2. The ignitrons are connected in the same manner as in the Fig. 2 system to the primaries 3533, 3&5, 397 of the Welding transformers 30|. The biasing potential for the firing thyratrons 327 to 33? is, however, derived directly from the buses and 3| 3 of the power supply and not through resistors 359 and 339 as in the Fig. 2 modification and is the same for all of the thyratrons.

The thyratrons are controlled from a Control Circuit, on the whole, similar to the Control Circuit of the Patent 2,619,591. The former Control Circuit includes a plurality of rheostats 4|?, 4|9 and iii which are supplied with potential from a delta transformer 423 connected to the main buses 309, 3l l and 3|3. These rheostats are connected in delta across midtaps of the secondaries 425, 427, 529 of the delta transformer. The midtaps are connected through resistors 43|, 433, 435 to a common conductor A137 which constitutes a neutral auxiliary bus. Potentials depending in phase position on the settings of the movable taps 439, 445, 443 of the rheostats 4|7, 4|9, 42|, respectively, may be derived between each of the taps and the bus rihe bus 437 is connected to the common junctions of the cathodes 445, 447 and 449 of one group of three thyratrons 45|, 453 and 455, respectively, and the anodes 55e and 45l of a second group of three thyratrons 463, 455 and 467, respectively. The anode 59 of the iirst thyratron 45| of the for-mer group is connected to one of the taps 443 through the primary 47| of a transformer 473; the anode of the second thyratron 453 is connected to a second tap 44| through the primary 477 of a second transform-er 479 and the anode liti of. the third thyratron 455 is connected to the tap |339 through a network consisting of a capacitor having a resistor 485 in parallel to it, and the primary 487 of the third transformer i3d rhe cathodes ASI, 433, 495 of the thyratrons et?, respectively, of the other group are correspondingly connected through the primaries 55N, respectively, of additional transformers 5st, 5557, respectively, to the taps 44e, et: and eas, respectively. The cathode 495 of the third thyratron 45'! of the last mentioned group is connected through a capacitor-resistor network 559, 5|| similar to the network. 433, 485 through which the third thyratron c55 of the first group is connected. In the control circuits of the thyratrons 45|, 453, 455 of the iirst group, a common bias 5 i 3 is connected. In the control circuits of the thyratrons 433, 45, 457 of the second group, separate biases are impressed from a common transformer i l 5 having a single primary and three seconda-ries which supplied from two main buses 3:59 and SIS. The biases 5|3 and SI5 are of a magnitude and polarity such as to maintain the thyratrons non-conductive.

The biases 5 |3 and 5|5 in the control circuits of the thyratrons may be counteracted by potential derived from the Sequence Timer and Frequency Determining Circuit H5 of the type shown in Fig. 3 of Patent 2,619,591, (Fig. 1C here) through line conductors 66 and 80, respectively. These conductors are identical to the conductors labeled 55 and 88 of the Fig. 3 circuit shown in application Patent 2,619,591. The counteracting potentials are impressed rst through conductors 65 at the beginnings of a predetermined number of successive periods of the supply, and thereafter through the conductors at the beginnings of a succeeding number of periods of the supply preferably equal to the first number.

The first pulse impressed through the conductors 63 charges a capacitor 5|? in the control circuit of the irst thyratron fll through a recti- Iier 5|@ to a potential such that the bias in this control circuit is counteracted. At an instant predetermined bythe setting of the associated rlieostat 42|, this thyratron is rendered conductive and current flows through the transformer 473 connected in its anode circuit. From one secondary 52| of this transformer, a pulse is impressed through a pair of conductors 523 in the control circuit of the firing thyratron 327 of one of the ignitrons 3|5. This ignitron is rendered conductive and current flows through the associated primary 39 of the welding transformer 3M. From another secondary 525 of the transformer 473 connected to the first thyratron 615|, a pulse is impressed through a rectifier 523i t0 charge a capacitor 529 in the control circuit of the second thyratron 53. At an instant predetermined by lthe setting of the associated rheostat 4 9, this second thyratron is now rendered conductive and current is transmitted through the associated transformer 479 in its anode circuit. From a secondary Winding 53| of the transformer 4:'9 through another pair of conductors 533, the firing thyratron 33| of a second ignitron 3| 9 is rendered conductive. Current now flows through the second primary 365 of the welding transformer 3FM. From another secondary winding 535 of the transformer 279 supplied from the second thyratron 53, potential is impressed through a rectiiier 537 to charge a capacitor 539 in the control circuit of a third thyratron 55. At an instant predetermined by the setting of the associated rheostat 4l 7, this third thyratron is rendered conductive. Current now flows through the transformer 489 connected in the anode circuit of this thyra tron and through the capacitor-resistor network 433, 485. From the secondary 54! of this transformer 439, through conductors 543, the iiring thyratron 335 of a third ignitrc-n 323 is rendered conductive. Current now flows through the third. primary 3&7 of the welding transformer 3l) l. The rheostats 42 I, 4| 9 and lll 'i are so set that the curansa-851 il rent ilo-rv through each o therespeetive ignitrons 3l5, 32:3 and is initiated preferably at the instant when the bus 358, 3l l and 3 i 3 connected to its anode becomes positive-relative to the other buses- The third ignitron 323 is conducting at the end ofthe first period or" the supply. At this time, the capacitor ses in the anode circuit of the thyratron 455L through Which the firing thyratron 335 of the third ignitron was fired is charged and is gradually discharging through the shunt resistor 1385; the discharge is at a relative low rate.

Another pulse is now impressed from the conductors 66. The rst and second thyratrons d5! andY 453 and their associated ignitrons Sie and B19-are rendered conductive as during the first supply-period; the former in accordance with the settings'pf the rheostats 42| and lle and the latter at the instant when the buses tts and 3l I connected to the anodes of the ignitrons SSE and 319, respectively, becomepositive. The firing ci the third thyratron 355 is, however, this time not alone controlled by the'Y setting of the associated rheostat 4H. In addition to the potential irnpressed from the rheostat, the third thyratron now hase. potential impressed in its anode circuit wlichis dependent on the residual charge re maning on the capacitor 1%83. This charge has such a polarityT as to increase the negative potential of the anode il relative to the cathode 4491 andthus tends to counteract the positive potential impressed from the rheostat si?. The third thyratron is, therefore, rendered conductive etna greater interval followingr the instant whenl the second thyratron is rendered conductive than it was duringthe rst sequence of oper-- ations of the three control thyratrons G5i, fi, 455. During this additional time, the current ow: through the .second primary 3G15 of the Weldingtransformer 33! decays somewhat. At the endl o1y this intervalthereof, the anode cathode potential of the third thyratron 55A becomes positive and the third thyratron is rendered conductive firing the thirdiiring thyratron 335 andY the associated ignitron 325., and the current flows through the associated primary 3M of the Weldngtransformer.

The operation of they third thyratron circuit isv illustrated in Fig. 6. In this View, potential is plotted vertically and time horizontally. labeled the overlapping half eine waves represent the potentials impressed from the rheostats 42, M9, 417. The shaded areas under-the third and sixth sine waves represent the` time intervals during which the third thyratron sl55 is conductive. Thesawtoothrshaped curve represents thecontrol potential impressed from the transformer @le and which counteracts the bias thecontrol circuit of the third thyratron The. slopingV line extending from the peak of the -f third half sine wave represents the decay of the potential on the capacitor A83 inthe anode circuit of4 the third thyratron. The third half sine Waveisshaded throughout to indicate that the thyratron 455 is rendered conductive early duringV this corresponding half period. During the succeeding interval when ring potential is impressed on the third thyratron, the anode potentialY of the third thyratron is negative until the instant represented by the point at which the upper sloping linecrosses the sixth half sine wave; rThis instant is substantially later in the corresponding half period than is the corresponding instant When the third thyratron is rendered conductive for theVv iirst time.

lil

vother buses 3S andV 3H.

isf

Since the current flow through the second' primary 305 Was permitted to decay somewhat during the short interval during which the conductivity of the third control thyratron 455 was delayed, the current flow through the third primary 307 does not build up the current inthe transiomer 36| as abruptly as it would if thevv third thyratron 455 were fired early. The abovedescribed operation would be repeated if additional pulses were supplied through the conduc tors 66. For the purpose of 'simplifying the explanation, I shall assume that pulses are supplied only during tWo periods of the supply through the conductors 6G.

Pulses are now supplied at the beginnings of two succeeding periods through the conductorsV The rst of these pulses causes the rst thyratron it of the second group to become conductive. The current ilovv through this thyratron induces a potential in the control circuit of the firing thyratron 323 of a first ignitron 311- of the second group, and current now flows from left to right through the first primary 333. The timing of the pulse derivedY from the conduc- -tors 39 is such that the :first thyratron of` the` second group is iired approximately @A2 of a' supply period after the lower bus 313 connected to the anode of the thirdy thyratron 323 of the' first group becomes positive with respect to the Because of thedelay in the firing of the thirdigntron' 323; this ignitron is, atA this time, non-conductive. The rst ignitron 3V! of the secondv group accordingly alone conducts current through the rst primary 3133.

As the second and third thyratrons 435 and S-of'the second group are each, in its turn, reirdered conductive, the second andV third ignitrons 32| and 325 of the second group are nred and current is conducted from left toV right through the associatedprimaries 3%5 and lli'iiof the welding transformer. As was the case when the thirdk thyratron 1355 of the rst group Was rendered conductive, a capacitorilin theani- 0de circuit of the third thyratron 461 of the second group is now charged. the capacitor is slowly dissipated' through the` shunt'resistor El l.

Another pulse is now impressed through the conductors 3G and the first, second and third thi ratrons S3 and i655 o the Control Circuit are a-gainlrendered conductive and, in turn, cause the associated ignitrons 3H and 32H to become con'- ductive as before. (5%) potential eounteracts the rheostat (lill) potential and the third thyratron i6-1 is rendered conductive correspondingly later than it was the rst time. The associated ignitron 325 is rendered conductive correspondingly later and the current flow'through the transformer 3M deeays somewhat before it is further built up byv conduction through the third primary 30T.

A pulse isnow impressed through the conductors 5E at the beginning of a succeeding period of the supply. New the iirst ignitronv 3150i the A ignitron of each group is thus avoided. These ignitrons'now do notbecorne` so hot as to conduct The charge on` rihis time, the capacitor continuously and the transformer is not damaged.

The operation just described is illustrated graphically in Fig. 7. In this graph, current is plotted vertically and timed horizontally. As labeled each of the loops above the time axis represents the current conducted in succession by the three ignitrons Si d and 323 of the rst group. The loops below the time axis correspondingly represent the current conducted by the ignitrons 3V', tt and 325 of the second group. In each case the fifth loop 3 i and @2i is longer than the others and drops to correspond to the delay in ring of the third thyratron s55 and 1357 when they are lired for the second time. The sixth loop 523 of the first group intersects the time axis just to the left of the first loop 3 Il of the second group.

In the apparatus shown in Fig. 8, power derived from a single-phase source or" commercial frequency is supplied to a lood of substantially smaller frequency. This system includes a welding transformer 555 having a primary 553 provided with an intermediate tap. This tap is connected to one bus of the supply. The terminals of the primary 553 are each connected to another bus 55? of the supply through a pair of ignitrons 559 and 56E, and 553 and 5%?5 in anti-parallel. Each ignitron is fired rrorn a thyratron 5G?, 55S, Eli and 5!3, respectively.

When the apparatus is in the quiescent state, the thyratrons 55? to 573 are maintained nonconduotive by bias 5'55, 5l?, 5l9, 513i impressed in the control circuit of each respectively. The bias 5!5 and 59A impressed in the control circuit or" one of the thyratrons 55? and 5"!3, respectively, of each pair is substantially lower than the bias Ell and Els impressed in the control circuit of the other thyratrons 559 and 5l' l, respectively.

The thyra-trons are controlled from a Control Circuit and Frequency Determining Circuit and Sequence Timer Ets of the type shown in application Paten 3,5i.il,652. The transformers of the system shown in Fig. 8 which are identiiied by the numerals ill, 193, 239 and 243 are identical to the same numbered transformers of the system shown in Figs. i and 2 of Patent 2,619,591 (Figs. 1A and 1B).

The operation of the Control Circuit, Frequency Determining Circuit and Sequence Timer 55.23 is identical to that oi the same circuit shown in Patent 2,530,552. Pulses are supplied during a predetermined number of successive half periods of the supply alternately through transformer iii and transformer 93. These pulses are impressed in the control circuits of the thyratrons 55S and displaced in phase with reference to the potential supplied from the buses 555 and 55?. This phase displacement is illustrated as of the order of 90 in Patent 2,510,652 and may be the same in this case, The phasing is such that when any pulse is impressed the bus to which the anode ci the associated ignitron is connected is positive relative to the other bus. During a succeeding number of half periods of the supply, similar pulses are supplied alternately through transformers 239 and 2F55. The pulses supplied through 'transformers Vil, E53, 239 and 249 may he of approximately sine wave forno. The amplitude of each of the pulses is suiiicient to counteract the bias 5l5 to 58. However, the low biases 575 and are counteracted earlier in the half periods of the supply than the high biases 511 and 559.

When a pulse is impressed through the transformer l l i, the associated firing thyratron 561 is Cil has been interrupted.

rendered conductive early in the half period of the supply 555, 55T. The corresponding ignitron 559 is then fired and current is conducted from the upper bus 557 through the ignitron, the leithand portion of the primary 553 to the other bus 555. During the succeeding half period of the supply, a pulse is supplied through the transformer |93. The associated thyratron dii and its ignitron 553 are now fired substantially later in the half period of the supply, current is conducted from the lower bus 555 through the righthand portion of the primary 553, the ignitron to the upper bus 5.5i. Because the iiring of the second ignitron is delayed, the current in the transformer 55! built up through the first ignitron is permitted to decay somewhat in the saine manner as the current of the second ignitrons B2i and 325 of the systems shown in Figs. 2, 5 and 5A. Another pulse is now supplied through the transformer i ll and the first ignitron 559 is again rendered conductive early in its half period cf the supply. Current again iiows through the lefthand portion of the primary 553. Thereafter, a pulse is supplied through the transformer E83 and the second ignitron 563 is again rendered conductive later in its positive half period or" the supply. A. second current pulse now .dows through this ignitron 553 and through the right-hand portion of the primary 553. The alternate supply ci pulses through the transformers ill 259 may continue in this manner until a low requency half wave of the desired duration is supplied through the transformers 55 i. For the purpose of the present explanation, we assume that each half period of the low frequency is made up of four half periods of the supply frequency 555, 552i accordingly that only two pulses are supplied through each of the transformers lli and 53.

Pulses are now supplied through the transformers 239 and 249 in the control circuits of the associated ring thyratrons. The supplied through the transformer 23S renders the associated ring thyratron dit conductive early in its positive half period of the s' ply and current ows through the ignitron rihe ignitron 555 is iired at an instant in the half period of the high frequency supply determined by the phase of the pulse supplied from the transformer 239. This instant may be approximately /a of the supply period after the zero instant as illustrated Fig. 3 of app icaticn Serial No. 52,194. At this time the second .ignitron 553 of the first group has been conductive for approximately one supply period and the current flow through it Current now flows from the upper bus 55l through the conductive ignitron 555, the right-hand portion of the primary 553 to the lower bus 555. During the next half period of the supply, potential is through the transformer Zidi? in the control circuit oi the other nring thyratron Because of the high 'cias 5H', this thyre on is new rendered conductive later in its posi ve half period than the first thyratron ring the other ignition 5 5 l. Current now flows from the bus "55 through the left-hand portion of the primary 553, the ignitlon to the upper bus 55?. Because of the delay in firing of the second ignitron, the current flow through the Welding transformer 55! decays somewhat before it is further built up oy the current Iiow through the second ignition. Another pulse is now supplied thro` gli the transformer 233 and the first ignitron 55w is rendered conductive earlier in its half period than the second igm'tron conducting current through the right-hand portion of the primary 553. A fourth pulse is now supplied through the transformer Eile and current is again conducted through the left-hand perdon of the primary 553 and the second ignitrcn tti. Thereafter, in dependence upon the setting of the sequence timer, the abovedescribed process may be repeated.

D ing the above-described process, current is supplied first from left to right through the portions of the primary for our half periods o the supply and then trom right to left through the primary during four succeeding half periods. In each case, the current flow through the last conductive ignitron 63 or 5%! is interrupted before the current flow through the rst'tube oi the second group is initiated. This interruption is effected by the delay in tiring the second ignitron in each case.

rIhe apparatus shown in Fig. 9 is similar to the apparatus shown in Fig. S. Elowever, in the system shown in Fig. 9 the biases 585, ii'i, 589 and lii in the control circuits of the thyratrons et? to are alike. The latter thyratrons are controlled from a Control Circuit, Frequency Determining Circuit and Sequence riirner 5%3 similar to that shown in Patent 2,519,652 except for the anode circuits of the thyratrons I and 25 which are identical to the like numbered thyratrons of the system shown in Patent 2,5i0,652. In the anode circuits of the thyratrons IES and 25 (identical to the thyratrons 23 and 25 of Patent 2,510,652) of the system shown in Fig. 9, networks are connected, each consisting of a capacitor 95 and te? shunted by a resistor 59S and Bei, respectively. These networks are similar to the networks 433, 285; and 5%, 5H included in the apparatus shown in Fig. 5A of the present application. As in the system shown in Fig. 5A, these networks 5&35, 5S?, Elli are in series with the primaries ISS and @t3 oi the transformers it and i, respectively.

ln the same manner as in the Fig. 3 system, a pulse is supplied first through the transformer iii to render the irst ignitron 55S conductive early in its positive half period. Current is conducted through the left-hand portion of the primary 553. The thyratron 23 connected to the primary 94 of the transformer E93 is now rendered conductive, a pulse is supplied through this transformer 6&3, t e second ignitron is fired early in its half period and current hows through the right-hand portion of the primary.

At the carne time, the capacitor in the anode circuit of the thyratron E23 is charged. This capacitor discharges slowly through its shunting resistor A second pulse is now impressed through the transiorrner i'H and the first ignitron is again fired conducting current through the lett-hand portion of the primary, Potential is now again impressed to re the thyratron 23 from the secondary winding EBI of the tra -sforrner i'Ei, but because of the counter-potential, on the capacitor 555, this thyratron is fired substantially later in its positive half period than the first time. rIhe associated ignitron 553 is fired correspondingly later and the current flow through the primary decays somewhat before it is further built up by the flow through the latter ignitron.

Potential is now impressed through the transformer 23S to fire the rst ignitron its or" the second group. Because of the delay in the ring of the second ignitron 5%53 of the first group, the current flow through the latter is interrupted before the rst ignitron 565 of Vthe second group fires. The second ignitron 55H o the second group is now fired during the subsequent half period of the supply as early in its positive half period` as the rst ignitron etti. The charge on the capacitor 5S? in the anode circuit of the second thyratron 25 is now built up. The first ignitron t of the second group is now -fired for the second time and, thereafter, the second ignitron 555i is fired. But because of the charge on the capacitor 59'?, 'the latter ignitron 5%! is ired substantially later in its haii period than it was the nrst time so that the current in the transformer 55! now decays somewhat before it is further built up by the current flow through the ignitron. rihe above-described process may be repeated in accordance with the requirements of the material to be welded and the setting of the sequence timing. During each repetition, the second ignitron 57i or tti oi the last conducting group becomes non-conductive before the first ignitron of the newly conducting group rires.

In the apparatus .hewn in Figs. 1c and 10A, power derived from a polyphase commercial source is supplied to a welding transformer E65 at a substantially lower frequency than that of the source. The transformer includes a pair of primary windings @Iii and i; the rst @el connected between the upper ous :il i oi the suppi and the center bus Si through a pair of ignitrons i 5 and Si? in enti-parallel; the second connected between the center bus S43 and the lower ous through a second pair ci ignitrons 552i and in anti-parallel. With each or" these ignitrons G i 5, i "i, i, @23 a thyratron i325, and. i, respectively, is associated. The firing thyratrons 525i to tt are biased to non-conductivity and in the quiescent state ci the apparatus, the ignitrons Sie, $311, iti, are non-conductive.

The ring thyratrons are controlied from a Control Circuit including iour thyratrons 533, 635, et?, 39, each associated with one of the fu*- thyratrons to t3 i respectively. The Control Circuit i5 supplied from the secondary winding of the delta transformer 5M energized from the inain buses Sii, LHS, rEhe center taps of the windings of this secondari' are connected together, each through a resistor wie, gli?, respectively. The conimon junction of the resistors constitutes a ne 'tral bus for the Control Circuit. The center tap of one winding of the secondary is connected to the common iunotion of a pair of rheostats $53 and i555. The rein ining terminals of the r'ncostats are connected to the other center taps. The anode 55'! ci the control thyratron associated with one ignitron sie is connected to the adjustable tap oi' one rheostat through the primary @El a transformer The cathode SL.. of a control thyratron S35 associated with the anti-parallel connected ignitron 5 i is connected to the saine tap tt through the primary et? of another transformer The cathode @il of the former thyratron 533 and the anode @i3 of the latter t connected to the neutral ous ii. The anode tie and cathode El? of thyratrons ttl and 539, respectively, associated with the otr er ignitrons 552i and 21.- are connected to the adjustable tap 5?9 of the other rhecstat through the primaries @Si and E, l spectively, or" another pair or transformers 25 and espectively. The cathode and anode te! of these thyratrous t3? and $39,' respectively, are connected to the neutral bus 535i. A common bias 693 is connected in the control circuits of the two control thyratrons 633 and 631. The other two control thyratrons 635 and 639 are provided in their control circuit with separate biasing potentials from a transformer 695 having a single primary and a pair of seoondaries. The bias impressed in the control circuit of each of the control thyratrons 633 and 63: is sufficient to maintain the control thyratrons non-conductive in the quiescent state of the apparatus.

The conductivity of the control thyratrons is controlled from a Sequence Timer and Frequency Determining Circuit 696 of the type shown in Fig. 3 of application Patent 2,619,59i. The pairs of conductors identified by the numbers et and 80 in Fig. 10A of the present application are identical to the conductors identified by the numerals 66 and 86 in Fig. 3 of the application Patent 2,619,591.

in the operation of the apparatus, pulses are Supplied rst through the pair of conductors 65 at the beginnings of a predetermined number of successive periods of supply and then through the conductors S6 at the beginnings of a succeeding number of successive periods of the supply. A pulse derived from the conductor 66 is impressed through a rectifier 691 across a capacitor 636 connected between the control electrode 10i of the first control thyratron 633 and the bias SaS. This capacitor 699 is charged in such a sense as to counteract the bias 10i. At an instant predetermined by the setting of the associated rheostat 655, the thyratron 633 is rendered conductive and current flows through the primary 66! of the associated transformer 653. Firing potential is now impressed through conductors 763 from a secondary winding TG5 of this transformer in the control circuit of the firing thyratron 625 for the first ignitron 6 I 5. The rheostat 655 is so set that this potential is impressed at the instant when the bus 6I I connected to the anode of the ignitron SI5 becomes positive relative to the other buses. This ignitron, therefore, conducts current through its associated primary 507 of the welding transformer 605.

From a second winding 101 of the control transformer 663, potential is impressed across a capacitor 169 connected in the control circuit of the second thyratron 631 in the same manner as the capacitor 699 is connected in the control circuit of the nrst thyratron 633. At an instant predetermined by the setting of the associated rheostat 653, the second thyratron 631 is now r-endered conductive conducting current through its associated control transformer 685. From the secondary TII of this transformer 685 through conductors H3, the associated firing thyratron 629 is rendered conductive. This thyratron is rendered conductive at the instant when the bus 6I3 connected to the anode of its ignitron 62| becomes positive relative to the other buses. Current is now conducted through the second winding 609 of the welding transformer. This current continues to iiow for a substantially longer time interval than the current through the first primary 601 and the current built up in the welding transformer 665 by the conduction of the two ignitrons SI5 and 62! decays somewhat. At the beginning of the succeeding period of the supply, a second pulse is supplied through the conductors 6B. The first thyratron 533 is again rendered conductive firing the first ignitron 6l5. Again current is transmitted through the rst primary 601. When the bus 6I3 connected to the anode of the second ignitron 62I again becomes more positive than the other buses, the second ignitron 62! is again ren- 18 dered conductive and current flows through its associated Winding 669.

If the sequence timer (696) is properly set, another pulse may be transmitted through the conductors 66 at the beginning of a further period of the supply. However, for the purpose of the present explanation, we may assume that only two pulses are transmitted through conductors 66 and, thereafter, two pulses are transmitted through the conductors 86 at the beginning of subsequent period of the supply. The nrst pulse is transmitted through conductors a short time before the instant when th-e bus 6I3 to which the anode of the rst thyratron 6H of the second group is connected becomes positive relative to the other buses SII and 6I9. The latter instant is approximately one period of the supply after the second ignitron 62I of the first group was rendered conductive. During this time, the current ow through the welding transformer 605 decayed, and the latter ignitron 62| became non-conductive.

The pulse transmitted through the conductors 80 is impressed on a capacitor 7I5 connected between the control electrode l'I'i and the cathode 665 of the first thyratron 635 of the second group through the bias 693 charging the latter to a potential such that the bias is counteracted. This thyratron 635 is now rendered conductive transmitting current through the primary 661 of its associated control transformer 669. From one secondary winding 'IIS of this transformer through conductors "I'ZI, the firing thyratron 62T associated with the ignitron Ell of the second group is rendered conductive. This ignitron now conducts and current of a polarity opposite to the polarity of the current which has been owing is transmitted through the rst primary 601. Through a second winding 'i23 of the control transformer 669, a potential is impressed to counteract the bias in the control circuit of the second control thyratron 639 of the second group. At an instant predetermined by the setting of its associated rheostat 653, this thyratron now becomes conductive rendering the associated ring thyratron 63| and its ignitron 623 conductive. Current now flows through the second primary 509. The current flow through this primary continues until the beginning of the succeeding period of the supply when a second pulse is transmitted through the conductors 86. The second period of the supply begins approximately a of the period of the supply after the second ignitron 623 was rendered conductive. The current iiow through the transformer 605 therefore decays somewhat before the second pulse is impressed through the conductors 80. The second pulse again charges the capacitor I I 5 in the control circuit of the first thyratron 635 rendering the latter conductive at the instant predetermined by the setting of its associated rheostat 655. The associated ignitron 6I5 is now again rendered conductive further building up the current in the transformer SDS. At an instant predetermined by the setting of the other rheostat 653, the second control thyratron 639 is rendered conductive and its associated ignitron 623 further conducts current through the second primary of the transformer. Now, the above-described process may again be repeated and current supplied to the welding transformer during another low frequency cycle. If it is repeated, the ignitron 623 will have become nonconductive before ignitron SI5 fires.

The operation of the system shown in Figs. 10 and 10A is graphically illustrated in Fig. 11. In

aeeassi thisview, current is plotted vertically and timed horizontally. As labeled the loops above the time axis correspond to the currentY conductedby the ignitrons SI and B2i; that is Ywindings @di and from right to left; and the loops below the axis correspond to the current conducted by the ignitrons til and 623 through the windings B'l and E99 from left to right. Current begins to flow through the primaryfl'l approximately le of a period of the supplyafterthe beginning of the periodfduring which the firstv pulse is impressed through the conductors 66. This current as indicated by the rst loop flows between the instants le and 1% ,after the beginningof period. At the instant T52- after the beginning of the period, the secondk ignitron ,E21 Vis rendered conductive and current owstthroiigh the winding E09 Aas represented by the second loop. This current flows until .1.3/12 of aperiodafter the beginning of the first period. APit this point,. current is again conducted through the first primary 5cl for approximately T42- of a period as represented bythe third loop G l tand then current flows through the secondprimary Gilt as represented bythe fourth loopptl. The current conducted by the second ignitronA now continues until just' before the first ignitronii il of the second group is rendered conductive. This current continues for somewhat less than one periodof the supply as represented by the fourthloop (52|. Thereafter, the current flowthroughtheother lignitrons as represented by thecorresponding loops l l, 623 below theA time axis.

VAlthough Lhave shown and,described certain specific embodiments of my invention, I am fully f aware that-niany modifications thereof are possible. Myinvention, therefore,v isnot to be restricted excepted insofar :asis necessitated bythe priorgart .and by the spirit of, theappended claims.

, I claim `asmy invention:

Lnpparatus;forconvertinggpower of one frequencyto'be-,derived from-,agpulsatingsupply to a substantially ,lower frequency to` be supplied ,to an inductivereactiveiload, including a plurality of electric Adischarge pathaeach consisting ofan anode and acathode means for, connectingY each saidlanodeand; cathode between said supply and said load; a firing circuitA for Aeach said. path; meansconnectingeach iring circuit toits associatedpath; andmeans .connected tosaid firing circuits .causingsaid ring circuits to fire said. paths periodically in ,a predetermined succession during successive pulsations ofsaidsupply; said appara- Vtus beingcharacterized .by the novelty cf a firing circuit for-at least thelast of .said paths to fire in said succession -including means for `firing said path later, during. at least one of theV pulsations duringY which it is fired, than the other paths are firedduring the pulsations during which they are fired.

Y2. 1n combination-.a plurality of conductors; a highly inductive. reactive transformer having a plurality of primaries each primary corresponding to oneiof:said conductors ;1a pair of-electrio discharge paths, each consisting of an anode and a cathode and having -a control. electrode interposed in anti-parallel between each conductor and its associated primary;I rst means connected to said control electrodes of said paths and including ybiasingmeans vfor supplying ya bias potential to biasY each of said paths. to., non-conductivity; Aand second means connected tosaid paths and including means for-supplying .potential pulses to counteract said ,biasing` potentials of each of paths in succession; saidrst means being .adapted to sup- Ell plysubstaiitially equal biasing potentials to all said paths, and said second means being adapted to supply said counteracting pulses in trains, said second means including a network operative to delay certain of said pulses so that the time interval between the (1i- 1) th pulse and the nth pulse of any train is substantially shorter than the time interval between the (cm-l) th and the mth pulse of any train where n is the number of said primaries and a is an integer greater than 1.

3. Apparatus for controlling the supply of current from an 1L phase supply to a highly inductive reactive transformer having n primaries, comprising in combination, means including at least one electric discharge path, consisting' of an anode and a cathode and having a control electrode, for connecting each primary to a corresponding phase; and means connected to the control electrode of each of said paths for rendering said paths each in its turn conductive periodically during successive pulsations of said supply, the means connected to the control electrode of the one of said paths which is rendered conductive the last during each period of conductivity of said paths including a network and means or actuating said network to operate selectively for delaying the edectiveness of said means connected to control electrode of said one path thereby rendering said one path conductive at later instants in the pulsations impressed on said one path, during at least one pulsation of said supply during which it is conductive, than the other paths.

4. Apparatus for controlling the supply of current from an n phase supply to a highly inductive reactive transformer having n primaries, comprising in combination: means including at least one electric discharge path, consisting of an anode and a cathode and having a control electrode, for connecting each primary to a corresponding phase; and means connected to the control electrode of each path for rendering said paths each in its turn conductive periodically during successive pulsations of said supply, each path being rendered conductive at a predetermined instant in correspondingl pulsation of said supply which instant occurs at a predetermined duration after a reference instant in said pulsatiomsaid predetermined instant oocurring at the same Vduration after said reference instant for all of said paths except the last one of said paths to conduct during each period of conductivity of said paths, the meansiconnected to the control electrode of the said one of said paths including a delaying network and means for actuating said network to operate selectively for rendering said one path conductive' at vsubstantially the saine duration after the reference instant of its corresponding pulsation as the other paths during the first pulsation of said supply during which it is conductive but at substantially a longer duration afterthe reference instant of its corresponding pulsation than the other paths during those pulsations during which it is conductive which'followthe iirst.

.5. En combination a plurality of conductors yadapted to be connected to an alternation current sup-ply; .ahighly inductive reactive .transformer having a plurality o primaries each'primary corresponding to one of said conductors; a pair of electric discharge paths each consisting of .an anode and a cathode andhaving acontrol ,electrode interposed in anti-parallel between eachconductor and its associated primary; first means vconnected to .theA control .electrodes Vof said paths-and including; biasing means for-supplying 

